Hitherto, in order to achieve reduced fuel consumption of automobiles by improving characteristics of tire, many investigations have been made with respect to tread rubber portion which causes 50 to 60% of hesteresis loss of tire. Due to recent developments with respect to polymer and carbon black and improvements with respect to blending technique including them, the hesteresis loss of the tread rubber portion has been decreased to 30 to 40% and thereby, rolling resistance of tire is also decreased remarkably.
It has been discovered, however, that even if it is tried to decrease hesteresis loss further only through the tread rubber portion, there is a certain limitation from viewpoints that wear resistance, steering stability and particularly grip property on wet road (hereinafter referred to as "WET property") are lowered.
On the other hand, although there are a lot of proposals that silica is used as reinforcement for the tread rubber portion, there is a problem, for example, that electric resistance of tire increases on increase of an amount of silica to be used.
As a result of the inventor's intensive study by considering the above-mentioned problems, the inventor of the present invention gave his attention to tire sidewall portion and has found that rolling resistance of tire can be lowered without lowering wear resistance and WET property of the tread portion and without increasing electric resistance of a whole tire by using the particular kind of carbon black as carbon black for reinforcement of the sidewall portion, replacing a part of the carbon black with the particular kind of silica, and particularly using the specific amount of silane coupling agent together.
Namely, an object of the present invention is to provide a rubber composition for tire sidewall which gives a tire having low rolling resistance, excellent wear resistance and WET property, and low electric resistance.